Messages in this thread | | | Date | Thu, 16 Dec 2010 19:42:29 +0100 | From | Oleg Nesterov <> | Subject | Re: [RFC][PATCH 5/5] sched: Reduce ttwu rq->lock contention |
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On 12/16, Peter Zijlstra wrote: > > +static int ttwu_force(struct task_struct *p, int wake_flags) > +{ > + struct rq *rq; > + int ret = 0; > + > + /* > + * Since we've already set TASK_WAKING this task's CPU cannot > + * change from under us.
I think it can. Yes, we've set TASK_WAKING. But, at least the task itself can change its state back to TASK_RUNNING without calling schedule. Say, __wait_event()-like code.
> +static int > +try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) > { > - int cpu, orig_cpu, this_cpu, success = 0; > + int cpu = task_cpu(p); > unsigned long flags; > - unsigned long en_flags = ENQUEUE_WAKEUP; > - struct rq *rq; > + int success = 0; > + int load; > > - this_cpu = get_cpu(); > - > - smp_wmb(); > - rq = task_rq_lock(p, &flags); > - if (!(p->state & state)) > - goto out; > + local_irq_save(flags); > + for (;;) { > + unsigned int task_state = p->state; > > - cpu = task_cpu(p); > + if (!(task_state & state)) > + goto out;
Well, this surely breaks the code like
CONDITION = true; wake_up_process(p);
At least we need mb() before we check task_state the first time.
Oleg.
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